While looking through the Apple App Store this morning I found this really useful little app - Paint Pot.
Effectively it is a database of hobby paints which can be used to create lists from. It covers all the major paint manufacturers - Citadel, Vallejo, Privateer Press etc - and appears to be up to date and current.
I was looking at it to create a shopping list so that I can know what paints I need to buy when I go to the hobby shop or make an Internet order.
However you can also use it as a record of colours you used on specific models or armies. This is illustrated in the next screenshot.
You can create a paint log for the model including a breakdown of specific mixes.
And best of all - it is free!